The present invention relates to a method of controlling atom transfer radical polymerizations.
Various living polymerization techniques have so far been developed and it has become possible to produce polymers controlled in molecular weight, molecular weight distribution and terminal structure. As examples, there may be mentioned the anionic coordination polymerization of polypropylene glycol and the living cationic polymerization using an iniferter and a Lewis acid catalyst. In addition, in recent years, the technique of living radical polymerization has been developed, which makes it possible to control the radical polymerization, which has so far been regarded as very difficult to control.
Living radical polymerization is a radical polymerization in which the activity of the polymerization terminus is not lost but is maintained. While, in its narrow sense, the term xe2x80x9cliving polymerizationxe2x80x9d means the polymerization which proceeds while the terminal activity is maintained, it generally includes the so-called pseudo-living polymerization in which terminally inactivated species and terminally active species are in equilibrium. It is the latter definition that applies in the present invention. In recent years, living polymerization has been energetically studied by a number of groups. As examples, there may be mentioned a technique using such a radical scavenger as a cobalt-porphyrin complex (J. Am. Chem. Soc., 1994, 116, 7943) or a nitroxide compound (Macromolecules, 1994, 27, 7228) and the atom transfer radical polymerization (ATRP) technique using an organic halide as an initiator and a transition metal complex as a catalyst, among others. Atom transfer radical polymerization is generally carried out using an organic halide or sulfonyl halide compound as an initiator and, as a catalyst, a metal complex containing a central metal atom selected from among elements of the groups 7, 8, 9, 10 and 11 of the Periodic Table (see e.g. Matyjaszewski et al., J. Am. Chem. Soc., 1995, 117, 5614; Macromolecules, 1995, 28, 7901; Science, 1996, 272, 866; or Sawamoto et al., Macromolecules, 1995, 28, 1721). According to these techniques, the rate of polymerization is generally very high, and, in spite of the fact that they involve radical polymerization in which such a termination reaction as coupling of radicals readily occurs, the polymerization proceeds in a living manner to give a polymer with a narrow molecular weight distribution (Mw/Mn=1.1 to 1.5), and the molecular weight can be arbitrarily controlled by selecting the monomer/initiator charge ratio. In the present specification, the term xe2x80x9cmolecular weight distributionxe2x80x9d means the weight average molecular weight/number average molecular weight ratio as determined by gel permeation chromatography.
As the term xe2x80x9catom transfer radical polymerizationxe2x80x9d indicates, an initiator-derived halogen atom generally occurs at the growing polymer terminus. In actuality, however, it is a problem that such atom may disappear owing to various side reactions.
Of the catalysts useful in atom transfer radical polymerization, some are completely soluble in the polymerization system and give homogeneous systems but most of them are not completely soluble, hence are used in heterogeneous systems. For example, when 2,2xe2x80x2-bipyridyl, one of the ligands in most frequent use, is used in the polymerization using CuCl or CuBr, the polymerization systems generally become heterogeneous. As a measure for obtaining a homogeneous system, there is a technique involving substitution of an alkyl group on the pyridine rings of bipyridyl and it is reported that substitution of 1-butylpentyl or the like results in formation of a homogeneous system. Further, it is reported that the use of a highly polar solvent such as ethylene carbonate results in an increased solubility of the complex to give a system more close to a homogeneous one (Macromolecules, 1998, 31, 1535). However, it is also mentioned that, even in that case, a reduction in solvent amount leads to a decreased solubility and a reduced rate, for instance.
It has recently been reported that aliphatic polyamines (e.g. pentamethyldiethylenetriamine), which are inexpensive and commercially available, are also effective ligands and can be used in lieu of bipyridyl ligands and the like. However, even the use of such ligands cannot render the polymerization system completely homogeneous.
If the polymerization system is heterogeneous, the catalyst may precipitate and/or stick to the vessel wall, so that it is not easy to stabilize the rate of polymerization and it is difficult to control the rate of polymerization because of the changing catalyst concentration.
On the other hand, the use of acetonitrile as a solvent is mentioned as an example in a patent specification (WO 97/18247), without mentioning any particular effect thereof. There is no description of the appropriateness of this for use in combination with aliphatic polyamine ligands. Furthermore, the relevant descriptions made therein are all concerned with the use thereof as a solvent. There is no description at all of the addition of acetonitrile or a nitrile compound in small amounts as an additive.
The initiation of atom transfer radical polymerization is generally effected by preparing a monomer/catalyst/solvent mixture and finally adding an initiator. When a liquid initiator is used, it can be added with ease using a syringe or the like. When it is a solid, it also can be added in the form of a solution in a solvent. Upon addition of the initiator, the polymerization begins to proceed immediately. Therefore, for obtaining a polymer with a narrow molecular weight distribution, it is necessary to add the initiator all at once. However, if the initiator is added all at once and the polymerization begins to take place immediately, a considerable heat liberation will be encountered. In large-scale production, this heat liberation is very dangerous. For avoiding this problem, a method is conceivable which would comprise adding the catalyst last after preparing a monomer/initiator/solvent mixture. In this case, catalyst addition can be made while watching the state of polymerization initiation, whereby the danger in question may be avoided. As far as the catalyst is concerned, unlike the case of the initiator mentioned above, adding the same over a certain time period theoretically does not give a remarkable influence on the molecular weight distribution and the like. However, the technique of atom transfer radical polymerization most often uses a metal complex, which is a solid, as the catalyst. Moreover, many a catalyst gives a heterogeneous polymerization system as mentioned above, and it is not easy to dissolve it in a solvent. It is, therefore, not easy to initiate the polymerization by addition of catalyst. In fact, no report has been made so far concerning such a process involving the addition of a catalyst in this manner.
In living polymerization, a growing terminus retains the polymerizing activity from the initial to terminal stage of polymerization and, as a result, the rate of polymerization shows an approximately linear relation with the monomer concentration. When living polymerization is carried out batchwise by charging the reaction apparatus with the whole amount of the monomer to be used in polymerization from the beginning, the amount of the monomer polymerized per unit time is greatest at the early stage and then gradually decreases as the monomer is consumed. Similar problems are encountered even in semi-batchwise polymerization, which is conducted by supplementing the monomer batchwose or continuously after initiation to avoid the risk of uncontrolled progress of polymerization, which is a matter of particular concern in radical polymerization. In this case, even if the amount of the monomer remaining in the polymerization system is maintained at a constant amount, the growing terminus concentration and catalyst concentration are highest at the early stage and then diluted with the accumulation of the polymer formed. As a result, like in the case of batchwise polymerization, the amount of the monomer polymerized per unit time is the greatest in the early stage and then decreases gradually. This amount of monomer polymerized per unit time determines the amount of heat liberated and, therefore, how to control and stabilize this heat liberation is very important in industrial polymerization processes. However, in such living polymerization as mentioned above, for the reasons mentioned above, it is usual that an intense heat liberation takes place in the early stage. This is an obstacle to scale enlargement and product structure control. If the catalyst activity is reduced to suppress this heat liberation, an undesirably long total polymerization time may be required. While the productivity is a very important factor in industrial scale production, a dilemma is encountered that enhancing the catalyst activity for curtailment of the total polymerization time results in excessive heat liberation in the early stage.
Accordingly, it is the object of the present invention to allow the terminal halogen atom to remain at a high rate in atom transfer radical polymerization, solve such problems as the difficulty in polymerization rate control as caused by catalyst precipitation and wide variation in catalyst amount, provide a simple and safe method of polymerization initiation as well as a method of controlling the rate of polymerization, and indicate a method of improving the polymerization procedure.
The present invention relates to a polymerization method wherein the atom transfer radical polymerization of a vinyl monomer is carried out under at least one condition selected from the group consisting of the following (1), (2), (3) and (4):
(1) in a substantially dehydrated system;
(2) in the presence of a nitrile compound;
(3) addition, to the system, of a ligand to the polymerization catalyst to thereby initiate the polymerization;
(4) varying a polymerization catalyst activity during polymerization to thereby control the rate of polymerization.
Atom transfer radical polymerization comprises equilibrium reactions involving the initiator, growing terminus and the transition metal complex catalyst but, basically, it is a radical polymerization involving formation of a radical at a growing terminus and monomer polymerization by means of the radical. Generally, as can be seen from the fact that emulsion polymerization and dispersion polymerization, for instance, are conducted in water in industrial productions, radical polymerization is not affected by water. In atom transfer radical polymerization as well, it is shown in the literature, including patent specifications, that emulsion polymerization and dispersion polymerization are possible. Further, there are descriptions to the effect that addition of water does not produce any problem but no report says that addition of water must be avoided. Judgment about the success or failure in polymerization control is generally made on the basis of the number average molecular weight and molecular weight distribution while almost no discussion is found about the residual terminal group percentage because of difficulty in determining the same, among others. Atom transfer radical polymerization can be carried out in the manner of bulk polymerization as well, and an example of the use of a distilled monomer is also disclosed in the literature, without any mention about the total water content on the overall basis including the catalyst and initiator or about the residual terminal group percentage, however, to say nothing of the case in which a solvent is used.
As a result of their intensive investigations, the present inventors found that the water content in the polymerization system is closely related with the disappearance of the terminal halogen atom and that polymers retaining the terminal halogen atom at a high percentage can be obtained by eliminating the water. Further, this technique is useful in those cases that a polar compound having relatively high hydrophilicity, such as a nitrile compound and/or a catalyst ligand, is used in accordance with the present invention.
As a result of their intensive investigations, the inventors further found that addition of a nitrile compound is effective in improving the diffusibility of the catalyst owing to its potential for coordination with a transition metal compound. For still more increasing this effect, the catalyst precursor transition metal compound such as CuBr is preferably admixed with a nitrile compound prior to addition of a ligand such as an amine.
The above effect obtained according to the present invention differs from the effect resulting from mere use of a polar solvent. When a catalyst, which renders the polymerization system heterogeneous, is used, the use of apolar solvent generally improves the solubility of the catalyst but the use of the solvent in a reduced amount brings about such results as a decreased polarity of the system as a whole, a reduced solubility of the catalyst and a reduced rate of reaction (Macromolecules, 1998, 31, 1535). On the contrary, the addition of a nitrile compound according to the present invention is effective even at a low addition amount. It does not merely improve the solubility of the catalyst but prevents the adhesion to the vessel wall and/or precipitation of the catalyst, which makes the system heterogeneous, to thereby contribute to achieve uniform catalyst diffusion under stirring. This technology is effective also in increasing the diffusibility of the metal complex or salt prior to ligand addition in the polymerization initiation by catalyst ligand addition which is to be mentioned next herein.
Furthermore, as a result of their intensive investigations, the inventors found out a method of initiating the atom transfer radical polymerization by addition of a ligand therefor. Thus, a ligand-free metal salt, such as CuBr, alone is added to the polymerization system and then a catalyst ligand is added, whereupon a complex is formed in the system and it exhibits catalyst activity to initiate the polymerization. Many of the catalysts for atom transfer radical polymerization give heterogeneous polymerization systems, as mentioned above, and it is not easy to add them as they are or in the form dissolved in a solvent to the system. On the contrary, many of the ligands themselves occur as liquids or are readily soluble in a solvent, hence their addition is easy. The metal complexes (salts) prior to ligand addition are in many cases poorer in solubility and diffusability than the metal complexes to serve as catalysts. Once such a metal complex (salt) has adhered to the vessel wall before ligand addition, ligand addition may not be accompanied by immediate complex formation in some instances. For preventing this, the above-mentioned addition of a nitrile compound is effective.
In addition, as a result of their investigations, the inventors found that the polymerization can be controlled by controlling the rate of polymerization by causing the catalyst activity to vary during polymerization. As the method of causing the catalyst activity to vary, there may be mentioned the method comprising adding the catalyst and the method comprising supplementing the transition metal complex (catalyst)-forming ligand, as for the above-mentioned initiation reaction. The transition metal complex to serve as a catalyst in the practice of the present invention is preferably a copper complex and, as for the solvent or additive, one capable of forming a complex with the transition metal but having no catalyst activity is preferably added.
The above-mentioned four conditions (1) to (4) as found out by the present invention are each independently effective in controlling atom transfer radical polymerization but, when combined, can lead to more pronounced effects.